Composite materials are used in ever increasing amounts in a wide variety of applications. For example, commercial aircraft are incorporating increasing amounts of composite materials into primary and secondary structure due to the favorable mechanical properties of composite materials. Such favorable properties may translate into a reduction in weight and an increase in payload capacity and fuel efficiency. In addition, composite materials may provide an extended service life for the aircraft as compared to aircraft formed of metallic construction.
Rework is occasionally required on composite structures in order to remove an inconsistency. An inconsistency may comprise a crack, a delamination, a void, a dent, porosity or other inconsistencies in the composite structure. An inconsistency may require rework when the inconsistency falls outside of desired tolerances. The removal of the inconsistency may require the reworking of an area in the composite structure containing the inconsistency by removing a portion of the composite structure containing the inconsistency and replacing the removed material with a patch. The patch may be formed as a stack of plies of composite material of the same or different type from which the composite structure is formed. The stacking sequence and fiber orientation of the composite plies in the patch may correspond to the stacking sequence and fiber orientation of the plies that make up the composite structure.
After assembling the patch from the stack of plies, the patch is typically bonded to the rework area with adhesive installed at the bondline between the patch and the rework area. Heat and pressure are typically applied to the patch such as with a heating blanket and a vacuum bag. The heating blanket may be used to elevate the bondline to the appropriate adhesive curing temperature. The vacuum bag may be used to consolidate the patch. During curing, the bondline may be held within a relatively narrow temperature range for a predetermined period of time in order to fully cure the adhesive. Furthermore, the entire area of the bondline may be held within the temperature range without substantial variation across the bondline.
Prior to bonding the patch to the rework area, a thermal survey may be required for the rework area. The thermal survey may be required to identify locations of non-uniform heating of the rework area by the heating blanket. Non-uniform heating may be caused by adjacent structure that may act as a heat sink drawing heat away from localized portions of the rework area resulting in differential heating of the bondline. In this regard, the thermal survey may provide a means for identifying hot and cold spots in the rework area such that adjustments can be made by adding temporary insulation to the composite structure and/or by adjusting the heating from the heating blanket until the temperature is within the required range.
A conventional thermal survey process may require assembling a surrogate patch that is a duplicate of the patch that is to be permanently bonded to the composite structure. In this regard, the conventional surrogate patch is formed of the same type of composite material and with the same number of plies as the final patch. Construction of a conventional surrogate patch is a time-consuming and labor-intensive process typically requiring hand-cutting of multiple composite plies each having a unique size and shape for each one of the rework area plies to be replaced. After the thermal survey, the conventional surrogate patch is typically discarded following a single use.
In addition to the thermal survey, a moisture removal process may be required to remove unwanted moisture from the rework area in order to improve the final bond between the patch and the rework area by reducing the risk of porosity within the bondline. A conventional moisture removal process comprises a drying cycle and may be required on composite structure that has been in service for a certain period of time and/or when certain adhesives are used in the repair process.
Unfortunately, the conventional drying cycle typically requires more than 24 hours to complete which may exceed the amount of time that may be available for rework operations performed in the field such as on in-service aircraft. Furthermore, the conventional practice of performing the thermal survey and drying cycle as two separate processes results in the application of two heating cycles on the composite structure which may affect the service life. Even further, the conventional thermal survey requires the labor-intensive and time-consuming process of fabricating the conventional surrogate patch after which the surrogate patch is discarded following a single use. In this regard, the materials for forming the composite surrogate patch may be relatively costly depending upon the amount and type of material used.
As can be seen, there exists a need in the art for a system and method for performing a thermal survey which obviates the need for fabricating a duplicate of the final patch. Furthermore, there exists a need in the art for a system and method for performing a moisture removal process on a rework area on composite structure that avoids the application of an additional heat cycle on the composite structure.